Stressed ferrite cores



Dec. 31, 1957 M. F. GOERTZETAL 2,818,514

sTREssED FERRITE coREs 2 Shets-Sheet 1 Filed oct. 2, 1952 WWA/rms. M.Fcomrz United States Patent O .STRESSED FERRITE CORES Matilda F. Goertz,'Jackson Heights, N. Y., and Howell J. Williams, Chatham, N. J.,assignors to Bell Telephone Laboltglie, 4vIncorporated, `New ork, N. Y.,Aincorporation of New York Application October 2, 1952, Seria,l'fNo.312,746

16 Claims. (Cl. S10-26) f ystrictive stresses. Substantial leior-t hasgone into the del `veloprnent of particular polycrystalline `fEerriteshaving substantially -zero magnetostrictive qualities.

We have found, however, that the high frequency magnetic properties,andrnore especially the ymaximum permeability andthe remnence of corescomposed `of mag-l netostrictive ferrites can be greatly improved bymaintaining them under proper stress.

Accordingly, one object of the invention is `to limprove the magneticcharacteristics of high frequency ferrite cores.

A ".fllhl .Obi iS to actively stress an annular magnetic core parallelto the closed magnetic circuit simply, inexpensively, and Withoutincreasing the high frequency electrical losses.

In one embodiment, an annular ferritic core having a chemicalcomposition of NioZnMFezO., is embedded in a plastic of a kind whichshrinks as it solidiiies, so that umform radial compressive stress ismaintained of a value such that the maximum permeability and remanenceare increased three and fourfold, respectively, and the hys=l teresisloop tends to become more nearly. rectangular.

Other objects and advantages of the present invention will'be apparentfrom the detailed description of certain specific devices illustratingthe principles of the invention,

which'lare shown in the drawings,

In thedrawings: Fig. l illustrates a ferrite core embedded in aconstric- ,tive type plastic; 5 Fig..2 is a cross-.sectional View of thedevice illustrated in Fig. l taken in a plane through 2-2 of thatligure;

Fig. 3 shows an alternative stressed core structure Vusing aconstrictiveplastic matrix;

Fig.. 4 represents a structure in which a ferrite'core -is stressed byhydraulic pressure;

Fig.,5 is a VView showing a cross-section of Fig. 4; Fig. 6 is aschematic circuit diagram illustrating the use of stressed ferritecores; and

Figs. 71, 8 and 9 are plots oi the magnetic properties of materialsunder stress.

Before commencing a detailed description of the draw` ings,the'develop-,ment of ferrite cores will be reviewed brieilytoaid` intheunderstanding of thepresent invention.

Although the cients recognized the magnetic properties of te,or'magnetic iron ore FeBOa, it was not until comparatively recently thatthe increased iinterest in high frequency circuits for manycommunications purposes led to ay Vdissatisfaction with the highfrequency losses of conventional metallic cores, and tok arenewed-interestv in the non-metallic magneticferrites. At the'beginningof this period of renewed interest, the values Patented Dec. 31, 1957 ofinitial permeability obtainable from the known ferrites werey asfollows:

Ferrite Initial permeability FeFeqOl (normal) Approximately 10. FeFe204(stresstreeL Approximately 70. CuEegOl (qu nched) Do.

MgF'egOr Maiinrnum approximately 10.

NlFenO4. f o

CoFegOl. Scarcely l.

MnFenO Maximum approximately 250 (but inconsistent) k line specimens,however, of a formula such as (XY) Fe204,

where. (XY) represents one molecular weight of two divalent metals, itlwas found that improvements in the foregoing values were possible. lnaddition, at the time when this investigation into the properties ofmagnetic oxides was begun, the theories of Ku-ssman, Becker, and Kerstenhad indicated that the absence of stress was an essential Conditionforhigh initial permeability and low hysteresisv loss (quoted from theabove-noted article by I. L. Snoek, one of the pioneers in this field).As the study of polycrystalline ferrites continued, it w-as found thatby combining proper proportions of ferrites having positive and negativemagnetostrictive properties, that is,

which respectively expand or contract in the principal di- Irection ofmagnetization, the magnetostriction of the polycrystalline constructurecan be reduced to almost zero, and the resultant core is substantiallystress-free.

The foregoing mode of attack, including the elimination of.magnetostrictive stresses, has, indeed, resulted in ferrite coreshaving suiciently improved magnetic qualities that they are nowextensively used for many high frequency applications. In accordancewith' the present invention, however, it has been determined that byvchoosing a polycrystalline ferrite having a suitable magnetostrictivecharacteristic (other than zero) and 'subjecting it to stress,l themagnetic properties of the ferrite will be greatly improved and' made'superior to those of ferrites made in the usual manner. Thus, althoughcertain isolated instances of the recognition that stresses causedslight changes in the magnetic properties of metallic ferromagneticmaterials may be found in the prior art, it is considered 'that thisstep, as applied to the present type of non-metallic ferrite cores, wasentirely contrary to the line of development which had been pursued bythose skilled in this ield. Furthermore, the preferred embodiment ofthe' invention, which is an annular ring of a particu'larpolycrystallineferrite embedded in a non-conducting constri'ctive plastic, is anexceedingly simple and inexpensive stressed core, having excellentmagnetic properties, and little or no eddy current losses at highfrequencies.

Referringmore particularly to the drawings, Fig. 1 illustratesv astressed ferrite core assembly in which the annular ferrite core 11 hasone or more copper windings l2 inductively linked therewith, and thecore and Windings are embedded in an insulating constrictive plasticbody 13. In accordance with one example of the invention, the core maybe made from a negative magnetostrictive nickel-zinc ferrite, thewindings from insulated fine copperV wire, and the plastic of amaterialv known commercially as Solectron 5003, which is a rigidstyrenepolyester casting resin which shrinks approximately nine percentduringl curing. While the foregoing materials have proved satisfactory,other suitable negative magnetostrictive ferrites and constrictiveinsulating materials may also be employed. In particular, othershrinking insulating materials which have shrinkage factors of the sameorder of magnitude as the nine percent mentioned above could be usedadvantageously. From one or two percent shrinkage factor, which would berequired to set up a useful stress in the core, up to about twentypercent, at which point the internal stresses make the body of theplastic break open as it cures, would be included in this range. Theapproximate dimensions of the stressed core assembly shown in Fig. 1 are2% inches diameter by 3715 inch thickness, with the ferrite core havinga diameter of approximately 3/1 inch and a crosssection approximately l;inch square. In addition, the coil or coils are normally of a relativelyfew number of turns or are moderately loosely wound in order to minimizeinterference with the constrictive effect of the plastic.

Fig. 2 is a cross-sectional view of Fig. 1 which shows the relativethinness of the core assembly, and the substantial radial and smallaxial extent of the enclosing plastic matrix. The purpose of thisdimensioning is to insure the application of sufficient radial stress onthe ferrite core, and to prevent axial shrinkage from interfering withthis radial stress. The uniform inward force on the annular core createsa strong internal compression of the core along its circular extent,coincident with its closed magnetic circuit. This compressionestablishes a direction of easy magnetization parallel thereto, and theannular core tends to assume the magnetic characteristics of a singlecrystal, having one axis of easy magnetization.

In the fabrication of the core assembly illustrated in Figs. l and 2, asuitable polymerizing catalyst is added to the casting resin. Theferrite core is then submerged in the liquid as by pouring the liquidover the core, and it is subjected to moderately increased temperaturefor several hours to accelerate the curing process. In one instance,using the Solectron 5003 mentioned above, a peroxide polymerizingcatalyst and a cobalt promoter,

the assembly was held at 45 C. for fifteen hours, at

80 C. for eighteen hours and at 120 C. for one hour, in successivestages. The foregoing times are merely i1- lustrative of the length oftime required for curing and shrinkage of the particular casting resin,and are by no means critical. When the promoter is not used, Somewhathigher temperatures and longer times are required. In addition, thecoils on the magnetostrictive core may be electrically energized in theproper direction to reduce the diameter of the core as the plasticcures, and

thus add to the deformation and stressing of the core.

The electrical current would be maintained until the plastic solidified,and, upon its release, the relaxation of the magnetostrictivedeformation would tend to place the core under stress even withoutshrinkage of the plastic.

The core assembly of Fig. 3 includes the ferrite core 16 embedded inconstrictive plastic matrix 17 with the core and plastic body being ofapproximately the thicknesses shown for the comparable elements of Fig.2.

, This type of core is suitable for magnetic coupling to one or morestraight conductors or a single-turn coils such as may be used in staticstorage arrays for digital computors and the like.

Figs. 4 and 5 illustrate an alternative arrangement for compressing thecore which was used in testing a number of annular core specimens. Inthe operation of this device, a ferrite core specimen 21 is lirstinserted into the recessed annular brass casing 22, and the threadedbrass retaining plug 23 is screwed into place. Hydraulic pressure isthen applied to the oil 24 within the rubber tube 25 which encircles thecore Z1. The nut 26 holds the oil inlet copper tube 2'7 securely inplace. In operation, various pressures may be applied to the ferritecore through a suitable gauged hydraulic pressure system, and themagnetic properties of the ferrite core tested by means of a coilthreaded through the open center of the unit.

Fig. 6 illustrates a schematic circuit diagram including a highfrequency source 31, a stressed ferrite core 32 embedded in a losslessinsulating matrix 33, and several coils 34, 35 and 36 linking the corewith the high frequency source and other electrical apparatus (notshown). When used in magnetic memory devices or in storage arrays thelow loss non-conducting ferrite core assemblies may be used at muchhigher pulse repetition rates than the metallic ferro-magnetic storageelements which have been widely used heretofore. Similarly, the upperfrequency limit of other devices such as magnetic ampliers may beextended by the use of these stressed ferrite cores.

The remarkable properties of stressed ferrite core assemblies inaccordance with the invention may be better appreciated by reference tothe plots of Figs. 7 to 9 and the following table:

. remanence (Br) may be noted in almost every instance where thespecimens are placed under compressive stress. The manganese zincspecimen, however, which has a positive magnetostriction as contrastedwith the negative magnetostriction of the other specimens, exhibits adecrease in remanence with compression, and would require tension toincrease its remanence. The stress applied to the ferrite cores shouldbe substantial, and increasing stress is found to improve theabove-noted magnetic properties up to the Verge of mechanical breakdownof the ferrite samples. One-half or even a lesser fraction of this forceis, however, suilicient to produce a marked improvement in the magneticproperties involved. As a concrete illustration of the foregoing, apressure of 300 pounds per square inch was found to nearly double theremanence of specimen #5, using the hydraulic device of Fig. 5.

Particularly good results have been obtained with ferrites of thegeneral formula NiolaZnwFezOg which may be seen to have good magneticproperties even when unstressed. The atomic percentages, using percentmetal as a base, would be lie- 66.67 percent, Ni-10.00 percent, andZn-23-33 percent in the case of a stoichiometric composition. Inaddition, it has been found that slight deviations of the iron contentare critical, and that quite an exact stoichiometric balance must bemaintained for best results. The balance between the nickel and zinc isnot as critical as that between the nickel and zinc together and theiron, Ybut stillmust be ymoderately close at a 3 :7 ratio,-for-bestresults. v

The characteristic curves ofthe stressed ferrite core specimen #1, whichhas the above-noted desirable ychemical composition, are shown inigs.'f7` and The `remanence'increases fromv 320 to I1460 gauss withcompression and the maximum permeability increases from 12,600 to36,800. The hysteresis loop of specimen 2, which has a chemicalcomposition that deviates somewhat from exact stoichiometric balance, isshown in Fig. 9. In this case the remanence increases from 900 to 1630gauss with applied stress, and the maximum permeability increases from7,900 to 12,300.

The foregoing data illustrates the effect of actively applied stress inmaking the hysteresis loops of certain ferrites more nearly rectangularand increasing their maximum permeabilities. Ferrite cores having theseproperties are adapted for many high frequency applications, with theformer characteristic being particularly useful in magnetic memorydevices and the latter in magnetic amplifiers.

One feature of the invention which should be reiterated is that thesolidifying of the constrictive plastic exerts enough force to producethe improvement in magnetic properties noted hereinbefore. While thecuring plastic might have exerted such a weak force that there would beno change in the magnetic properties, or such a powerful stress that thebrittle, ceramic-like ferrite cores would be cracked, we have discoveredthat suitable plastics will exert the proper force to greatly improvethe magnetic properties of the core without deleterious mechanicalfracture.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. An annular polycrystalline ferrite core, and a ring of constrictivematerial in engagement with said core and extending outwardly therefromwith the radial dimension of the constrictive ring from the core to theperiphery of said ring being substantially greater than its axialthickness.

2. An annular ferrite core having substantial negative magnetostriction,a plurality of conductive paths inductively coupled to said core, and arigid styrene-polyester resin encasing said core and applying stress tosaid core having a maximum compressive component in a circumferentialdirection in said core.

3. An. annular ferrite core, and an insulating constrictive body ofplastic material having a shrinkage factor in the order of magnitude ofnine percent extending outward radially from said core and formed toimpose a stress on said core which has a maximum component in acircumferential direction.

4. An. annular ferrite core, and a ring of insulating material extendingradially outwardly from said core and exerting an inward force on saidcore of a magnitude greater than fty pounds per square inch.

5. In combination, a high frequency source, a ferrite core coupled tosaid source, and means for imparting a stress to said core producing asubstantial compressive component in the direction of the principalmagnetic eld in said core which is greater in magnitude than anycompressive component in a direction perpendicular to said direction ofprincipal magnetic field.

6. In combination, a high frequency source, a ferrite core coupled tosaid source, and means including a plastic coating on said core forimparting a stress to said core producing a substantial compressivecomponent in the direction of the principal magnetic field in said corewhich is greater in magnitude than any compressive component in adirection perpendicular to said direction of principal magnetic field.

7. CrThe method of improving uthemagnetic characteristics of a highfrequency ferritecore which comprises preparing aninsulatingconstrictive plastic in liquid form, submer'ginghsaidcore'f'in the constrictive liquid in such a manner that the annulardistribution .ofA said liquid adjacent lthe. outer periphery of saidY,core substantially exceeds the ,radial dimension of said ,core and.the Aannular distribution of said liquid adjacent the inner peripheryof said core, and solidifying the liquid, thus placing the annular coreunder an active stress having a maximum compressive component in acircumferential direction therein.

8. The method of improving the magnetic characteristics of a highfrequency ferrite core which comprises preparing an insulatingconstrictive plastic in liquid form, submerging said core in theconstrictive liquid in such a manner that the annular distribution ofsaid liquid adjacent the outer periphery of said core substantiallyexceeds the radial dimension of said core and the annular distributionof said liquid adjacent the inner periphery of said core, solidifyingthe liquid, during the soliditication of said liquid energizing anelectrical circuit magnetically linked with said core in a direction toreduce the circumference of said core and deenergizing said circuit onsolidication of said liquid, thus placing the annular core under anactive stress having a maximum compressive component in acircumferential direction in said core.

9. In combination, a body of rigid constrictive insulating material, anda polycrystalline ferrite core embedded therein, said insulatingmaterial formed to produce a stress in said core having a maximumcompressive component in the direction in which the principal magneticfield is induced in said core.

l0. An annular ferrite core having an approximate chemical formula ofNioZna-,FezOg a plurality of conducting paths inductively coupled tosaid core, and means for applying a continuous stress to said core whichhas a maximum compressive component in a circumferential direction insaid core.

ll. In combination, a source of pulses of high repetition rate, and aferrite core coupled to said source, said core having substantialnegative magnetostriction, and said core embedded in a constrictiveplastic formed to impose on said core a maximum compressive stress inthe direction of the magnetic field in said core.

12. A stressed magnetic core having an approximate composition as givenby the chemical formula means for applying a magnetic iield to saidcore, and means including a rigid body of insulating materialpermanently associated with said core for maintaining said core understress having a major uniform component in the direction of saidmagnetic field.

13. An annular polycrystalline magnetic ferrite core of the chemicalformula Ni 3Zn0 -7Fe204, and means including a plastic insulatingcoating permanently associated with said core for applying to said corea stress which has a maximum component in a circumferential direction.

14. An annular ferrite core having substantial negativemagnetostriction, a coil coupled to said ferrite core, and an insulatingconstrictive matrix of a plastic material encasing said core and coiland formed so as to impose on said core a stress having a maximumcompressive component in a circumferential direction.

l5. A polycrystalline ferrite core, means for applying a magnetic iieldto said core, and hydraulic means for applying stress thereto having amaximum compressive component in the direction of the magnetic field insaid core.

16. An annular ferrite core, and an insulating constrictive matrix of aplastic material encasing said core, with the axial thickness of saidcore being substantially the same as that of said matrix, but with theradial extent of that portion of said constrictive matrix adjacent theouter periphery of said core being substantially greater ReferencesCited in the le of this patent UNITED STATES PATENTS Milton Dec. 30,1919 Potter Sept. 13, 1932 Abrams Aug. 15, 1939 Hill Jan. 21, 1947 SnoekOct. 26, 1948 Nesbitt et a1 Aug. 15, 1950 Robinson Oct. 24, 1950 CroninMay 12, 1953

1. AN ANNULAR POLYCRYSTALLINE FERRITE CORE, AND A RING OF CONSTRICTIVEMATERIAL IN ENGAGEMENT WITH SAID CORE AND EXTENDING OUTWARDLY THEREFROMWITH THE RADIAL DIMENSION OF THE CONSTRICTIVE RING FROM THE CORE TO THEPERIPHERY OF